This invention relates to a scanning and printing apparatus, such as a copier or facsimile machine, for flatbed scanning of documents and printing of document images.
A familiar example of such a scanning and printing apparatus is the electrophotographic copier in which a document is placed faced-down on a glass window called a copyboard, a cover is closed over the document, and the document is scanned by a movable scanning unit disposed below the copyboard. Traversing the document lengthwise, the scanning unit converts the image on the document to electrical signals which are sent to a printing unit comprising, for example, a rotating drum, a toner developing system, and a paper cassette. By extracting a sheet of paper from the cassette, transporting it under the drum, and transferring toner from the drum to the paper, the printing unit prints a black-and-white copy of the scanned image.
Color copiers comprising a thermal or ink-jet printing unit have also been developed. A typical color copier of this type scans a document three times, once for each of three primary colors, and transports the paper through the printing unit three times to have each color printed separately. Since the scanning speed is not in general equal to the printing speed, a buffer memory is needed to store the signals from the scanning unit before they are printed by the printing unit. It may further be necessary to process the signals sent from the scanning unit to the printing unit to compensate for differences between the spectral characteristics of the filters in the scanning unit and the inks in the printing unit.
A facsimile machine is similar in function, except that instead of sending electrical signals from its scanning unit to its printing unit, a facsimile machine sends signals from its scanning unit to a distant facsimile machine, and prints images received from a distant facsimile machine.
A problem of all these devices is their large size and high cost, which result from their complex internal structure. One cause of this problem is that the scanning unit and printing unit are independent, each having its own driving system. Many prior-art color copiers, for example, simply comprise a color scanner mounted atop a color thermal printer, the two units being completely separate except for the electrical interface between them.
In a copier, an associated problem is that to prevent distortion of the copied image, the speed of the driving system for the scanning unit must be accurately related to the speed of the driving system for paper transport in the printing unit. Expensive, high-precision stepping motors must therefore be used in these driving systems, and even so, distortion may occur due to vibration of the apparatus, or to motor irregularities.